From Hooman's notes:
Player Unit limits:
-------------------
Play	Veh	Bldg	Tot/P	Tot
-	---	---	---	---
1	200	250	450	450
2	160	175	335	670
3	115	100	215	645
4	90	75	165	660
5	77	50	127	635
6	65	48	113	678
Per player unit limit isn't hard set, and can be exceeded
by producing structure kits before deploying, or starting
production on vehicles before the limit is exceeded.
Current unit limit: 1023
[Includes mining beacons, fumaroles, disasters, etc.]
maxNumGaiaUnits = 1023 - 678 = 345
[assuming players don't build past their individual limits]
These values are pre-1.3.5; starting with 1.3.5 they were doubled. The table is soft unit limits.